Release coating compositions are used, for example, to coat papers or films which are to be used as a release backing, or a protective release sheet over the adhesive for pressure-sensitive labels, decals, tapes, etc. Labels which are provided with a pressure-sensitive adhesive still adhere to the release coated surface to a sufficient extent to enable the backing sheets with the adhesive labels thereon to be handled. However, it is essential that the labels can be readily separated from the release coated backing sheet without significantly reducing the adhesive forces on the back of the label. When such products are to be used, the release backing sheet is pulled off and discarded. The exposed pressure-sensitive adhesive is pressed onto a surface where the decal or label is to be placed. A common variety of release paper is one wherein one side of the paper (the release side) is coated with a polymeric siloxane material.
Silicones and silicone copolymers have been used extensively as release layers on paper, film, etc., because they are inherently low in surface energy. It is desirable that silicone-coated release papers and films have a release force which is low enough to enable the release paper to be easily removed from a pressure-sensitive adhesive-coated substrate but not so low that the release paper will become separated from the pressure-sensitive adhesive coating by forces normally encountered in the processing of the construction such as printing, die cutting and matrix stripping. "Release force" is defined as the amount of force required to peel or separate the release-coated substrate from the adhesive.
Polysiloxanes containing functional groups which can be radiation cured in the presence or absence of a photosensitizer have been described in various patents as useful silicone release compositions. U.S. Pat. No. 3,726,710 describes radiation-curable release compositions comprising an organopolysiloxane having olefinically unsaturated organic groups and a photosensitizer. The composition may be applied to a paper substrate and cured by exposure to high intensity radiation to produce a release coating.
U.S. Pat. No. 4,201,808 describes a release coating composition and release coated material which can be produced by applying to a substrate, for example, paper, a coating of a radiation-curable composition which comprises (a) from 10 to 90 weight percent of an organopolysiloxane containing an average of at least one acryloxy and/or methacryloxy group per molecule, (b) from 90 to 10 weight percent of a low molecular weight acrylyl crosslinker chosen from the group consisting of (i) di-, tri-, and tetra-functional acrylate or methacrylate esters of organic polyfunctional alcohols having a molecular weight of up to about 1200, and (ii) low molecular weight siloxane polyacrylates, and (c) from 0% to about 10% by weight of a photosensitizer.
The use of polysiloxanes with acrylate or methacrylate ester groups linked to SiC groups as radiation-curable coating materials for sheet-like carriers is described in, for example, U.S. Pat. Nos. 3,878,263; 4,064,286; 4,963,438; 4,908,274; 4,978,726; and 5,034,491. In the '274 patent, the polysiloxanes are prepared by reacting an epoxy-functional polysiloxane with acrylic or methacrylic acid. In the '726 patent, the polysiloxanes are obtained by reacting an epoxy-functionalized polysiloxane with an acrylic or methacrylic acid and a monocarboxylic acid free of double bonds capable of polymerizing. U.S. Pat. No. 4,963,438 describes acrylate containing polysiloxanes which are prepared by reacting polysiloxanes containing hydroxy-functional groups with a mixture of acrylic or methacrylic acid and a monocarboxylic acid which is free of double bonds capable of polymerizing.
It is desired to develop release coatings which can be applied to a substrate such as paper which is dimensionally stable and which remains flat (i.e., does not curl) under a variety of service conditions such as high humidity, elevated temperatures, freezer temperatures, etc., particularly when the release coated paper is used as a backing for a pressure-sensitive-coated laminate.